What it means
Every prescription plan keeps a formulary. Drugs on lower tiers usually cost less at the pharmacy. Drugs on higher tiers, or marked as needing prior authorization, cost more or take more paperwork.
A plan can cover a GLP-1 for type 2 diabetes and still exclude the same ingredient when it is prescribed for weight management. Many employer plans add or drop weight-loss drug coverage at renewal, so the list can change each year.
Search your plan's formulary by brand name, not just ingredient, and look for notes like PA (prior authorization), ST (step therapy) or QL (quantity limit).
If your drug is not on the formulary, ask whether the plan offers a formulary exception process. Your prescriber can request one with a letter explaining why you need that specific medicine.
Why it matters
Your formulary is the first thing to check before comparing list prices and savings programs.